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Groundwater investigation techniques 185

Fig. 5.37 Application of image well
theory in the case of pumping wells
affected by (a) constant head and (b)
impermeable boundaries. The sections
show the equivalent hydraulic system
required to meet the Theis solution
assumption of an aquifer of infinite
areal extent. After Ferris et al. (1962).

image well theory is given by Todd (1980) and in- for dilution. To be of use, an ideal tracer should be
cludes the case of a wedge-shaped aquifer, such as non-toxic and easily measured at very large dilutions.
a valley bounded by two converging impermeable The tracer should either be absent or present in very
boundaries. For complex regional aquifer situations low concentrations in the groundwater system to be
or for the analysis of multiple well systems, then a studied. An ideal tracer should also follow the same
numerical modelling approach to the solution of the pathway as the substance to be investigated, whether
steady-state or non-equilibrium groundwater flow particulates or solutes, and should not react chemic-
equations is usually required (Section 5.9). ally with the groundwater or be adsorbed on to the
aquifer rock.
There is no tracer available that meets all of
5.8.3 Tracer tests these criteria but there is a wide range of substances
and properties of water that can be used as tracers,
The principal applications for tracer tests are in the including: temperature; suspended particles (for
determination of groundwater flowpaths and resid- example, spores, fluorescent microspheres); solutes
ence times, in the measurement of aquifer properties (sodium chloride, halogen ions); dyes (fluorescent
and in the mapping and characterizing of karst dyes, optical brighteners); gases (noble gases, sul-
conduit networks. Much experience has been gained phur hexafluoride); microbes (bacteriophage); and
in karstic aquifers to demonstrate connectivity and environmental tracers such as chlorofluorocarbons
measure travel times. In intergranular aquifers, tracer (CFCs), radiocarbon, tritium, and the stable isotopes
tests are used less frequently because of the slower of hydrogen, oxygen, nitrogen, sulphur and carbon
groundwater velocities and the greater potential (Ward et al. 1998). The choice of tracer type will